Field of the Invention
The present invention relates to an image forming apparatus configured to form images on a recording material using an electro-photographic system or an electrostatic recording system.
Description of the Related Art
Hitherto, image forming apparatuses adopting an electro-photographic system are widely applied as copying machines, printers, plotters, facsimile machines, and multifunction machines having a plurality of these functions. In these types of image forming apparatuses, a configuration is known where a toner image formed on a photosensitive drum serving as an image bearing member is first primarily transferred to an intermediate transfer belt serving as an intermediate transfer body, and then secondarily transferred to a recording material, i.e., a sheet. An intermediate transfer belt in which a conductive agent is added to a resin material to adjust an electric resistance value to a desirable value is proposed.
In this type of image forming apparatus, classifications of sheets are defined by sheet type, such as sheet thickness, grammage and surface property, in order to correspond to various transfer materials. Further, an image forming apparatus is generally known in which a user designates the classification of the sheet being used for each sheet tray, in which transfer conditions and fixing conditions are changed according to the designated sheet classification. One example of change of transfer conditions is a case where the conditions are changed since a resistance value of the sheet differs greatly from a reference value, and toner cannot be transferred properly. In this case, for example, allotted voltage of a sheet may be adjusted to a setting corresponding to the resistance value of the sheet, or bias settings of a leading end portion and a trailing edge portion may be weakened compared to a center portion in order to prevent image defects caused at the leading end portion or the trailing end portion of the sheet. Further, even if the sheets belong to the same sheet classification, the electric resistance or other physical properties of the sheets may differ, and optimum transfer conditions and fixing conditions may not be obtained.
In order to cope with such condition, an image forming apparatus equipped with an adjustment function in which a user can set transfer conditions and fixing conditions on a user mode screen is known (Japanese Unexamined Patent Application Publication No. 2013-174875). One of such transfer conditions is a secondary transfer voltage. In that case, in the user mode, the user sets the secondary transfer voltage freely within a high voltage guarantee range of high voltage power supply, and a secondary transfer voltage corresponding to the set voltage is output from the high voltage power supply.
According to the image forming apparatus disclosed in the above-mentioned Japanese Unexamined Patent Application Publication No. 2013-174875, the user can set the secondary transfer voltage freely within the high voltage guarantee range of high pressure power supply, and the secondary transfer voltage set by the user is output from the high voltage power supply. There is a possibility that the secondary transfer voltage set by the user is set to an excessive voltage setting with respect to an impedance of the secondary transfer portion, due to reasons such as usage environment and durability. In this case, secondary transfer voltage is applied excessively from the high voltage power supply to the secondary transfer portion, and a current value exceeding the high voltage guarantee range may be supplied to the secondary transfer portion.
In order to cope with this problem, generally, a protection circuit is provided to a high voltage substrate in order to prevent heat generation or ignition, or to prevent failure caused by discharge within the substrate. Therefore, if a current value exceeding the high voltage guarantee range described above is supplied to the secondary transfer portion, the protection circuit is activated. Protection operations performed here may include an operation to switch to intermittent oscillation operation to prevent overcurrent from flowing continuously and cause heating, or an operation to switch off the high voltage output.
However, during such operation of the protection circuit, toner image on the intermediate transfer belt will not be transferred properly to the sheet, and a problem occurs in which image defects such as loss of image information may be caused in the end. In order to prevent such image defects caused by excessive voltage setting from occurring, it may be possible to set a uniform upper limit to the setting range of the secondary transfer voltage that the user can select. However, there are various types of sheets with different resistance values, differing in orders of a few digits or greater, even within the same sheet classification, or resistance values may differ in units of digits by moisture content, even if the sheets are of the same type, depending on the management condition of the sheets. Therefore, if the upper limit of the secondary transfer voltage is set in correspondence with a minimum resistance value, in a state where the resistance value is significantly greater than that value, the upper limit may be set to a value significantly lower than the true upper limit of the secondary transfer voltage that does not cause image defects. Thereby, the setting range of the secondary transfer voltage may be narrowed unnecessarily, and an optimum transfer condition may not be selected. Therefore, there is a demand for a system that enables the transfer voltage to be set by the user, while suppressing occurrence of image defects, without unnecessarily narrowing the setting range of the transfer voltage.
Meanwhile, there may be a case where the setting range is not reflected on the operation screen that the user uses for operation, such that the user is actually changing the set value on the operation screen, but the setting is not actually reflected and the transfer condition is not changed. There is a drawback in that the user checks the sheet in which the setting is not reflected in forming the image to optimize the transfer conditions, such that the workload is increased. Thus, there is a demand for a system that enables the transfer voltage to be set by the user, while suppressing occurrence of image defects, and reducing the workload related to optimizing the image quality.